Process for preparing spherical zinc oxide particles

ABSTRACT

The present invention provides a process for preparing ball-shaped zinc oxide particles and their use.

FIELD OF THE INVENTION

[0001] The present invention provides a process for preparing ball-shaped zinc oxide particles and their use.

BACKGROUND OF THE INVENTION

[0002] DE-A 199 07 704 discloses a process for the preparation of zinc oxides with a mean diameter of 5 to 30 nm and the formulation of these as concentrated dispersions in organic solvents and/or water by redispersion, wherein the dispersed zinc oxide is present substantially as isolated primary particles, i.e. agglomerate-free. Due to their fine state of division, these particles are outstandingly suitable as inorganic UV absorbers in transparent coatings or as coactivators for latex vulcanization.

[0003] Zinc oxides prepared in this way, as compared with those prepared by a calcination process, have the advantage that the primary particles are present in a non-agglomerated form or, if agglomerated to some extent, are reversibly agglomerated so they can be introduced and dispersed in gaseous, liquid or solid media in a homogeneous, primary particulate manner by means of appropriate measures. In contrast, in the case of thermally prepared particles, the primary particles grow together to give secondary particles or agglomerates with much higher particle sizes, due to the effects of heat, and these cannot be broken down into the primary particles again, either physically, mechanically or chemically, in an economic manner. Therefore the two types differ fundamentally, Which is why there are generally completely different, non-overlapping, areas of use for the two types.

[0004] It has now been found that, ZnO particles with spherical, ball-shaped surfaces can be obtained during preparation of the particles. These novel ZnO particles have improved vulcanization activity, as compared with zinc oxides which are prepared in accordance with DE-A 199 07 704 and lead to improved material properties for the vulcanizates.

SUMMARY OF THE INVENTION

[0005] The present invention is directed to a process for the batchwise preparation of zinc oxide particles including

[0006] A1) treating a methanolic solution for zinc salts corresponding to formula (I)

[0007] wherein

[0008] R represents H or a C₁-C₁₀ residue with a concentration of zinc ions (Zn) of 0.01 to 5 mol per kg of solution with

[0009] A2) a methanolic potassium hydroxide solution with a concentration of hydroxide ions of 1 to 10 mol per kg of solution

[0010] in a ratio of OH to Zn of 1.5 to 1.8 with stirring, the precipitation solution obtained after completion of the addition is matured at a temperature of 40 to 65° C. for a period of 5 to 50 min and is then cooled down to a temperature of ≦25° C.

[0011] The present invention also provides a process for the continuous preparation of zinc oxide particles including

[0012] B 1) mixing a methanolic solution of zinc salts corresponding to formula (I)

[0013] with a concentration of zinc ions (Zn) of 0.01 to 5 mol per kg of solution and

[0014] B2) a methanolic potassium hydroxide solution with a concentration of hydroxide ions (OH) of 1 to 10 mol per kg of solution

[0015] such that

[0016] b1) the ratio of OH to Zn is 1.5 to 1.8, and

[0017] b2) the mixture being formed in the mixing unit is blended homogeneously, i.e. to give a mixture with uniform density,

[0018] the precipitation solution formed is matured at a temperature of 40 to 65° C. for a period of 5 to 50 min and is then cooled down to a temperature of <25° C.

[0019] In addition, the present invention provides a process for the batchwise preparation of zinc oxide particles including

[0020] A1) treating a methanolic solution of zinc salts corresponding to formula (I)

[0021] with a concentration of zinc ions (Zn) of 0.01 to 5 mol per kg of solution with

[0022] A2) an optionally methanolic sodium hydroxide solution, which may optionally contain dissolved KOH, with a total concentration of 1 to 10 mol of hydroxide ions (OH) per kg of solution

[0023] in a molar ratio of OH to Zn of 1.5 to 1.8 with stirring, the precipitation solution obtained after completion of the addition is matured at a temperature of 40 to 65° C. for a period of 5 to 50 min and is then cooled down to a temperature of ≦25° C.

[0024] Further, the present invention also provides a process for the continuous preparation of zinc oxide particles including

[0025] B 1) mixing a methanolic solution of zinc salts corresponding to formula (I)

[0026] with a concentration of zinc ions (Zn) of 0.01 to 5 mol per kg of solution and

[0027] B2) a optionally methanolic sodium hydroxide solution, which may optionally contain dissolved KOH, with a total concentration of 1 to 10 mol of hydroxide ions (OH) per kg of solution

[0028] such that

[0029] b1) the ratio of OH to Zn is 1.5 to 1.8,

[0030] b2) the mixture being formed in the mixing unit is blended homogeneously, i.e. to give a mixture with uniform density,

[0031] the precipitation solution formed is matured at a temperature of 40 to 65° C. for a period of 5 to 50 min and is then cooled down to a temperature of <25° C.

DETAILED DESCRIPTION OF THE INVENTION

[0032] Zinc oxides prepared according to the present invention preferably have a mean primary particle size of 5 to 50 nm, more preferably of 5 to 20 nm. The particle size can be determined by ultracentrifuge measurements (H.G. Müller, Colloid. Polym. Sci., 267, 1113-1116 (1989)).

[0033] A substantial proportion of the particles, i.e. at least 60%, preferably at least 80%, more preferably at least 95% of the particles, have a spherical, ball-shaped surface, wherein the %-age data is calculated from the number of ball-shaped particles, with reference to the total number of particles found in a volume increment. The method used to determine this includes producing TEM images of the methanolic zinc oxide precipitates prepared in accordance with the present invention and evaluating the images visually. To prepare a sample, the zinc oxide precipitate or dispersion being investigated can be diluted about 1:1000 with a mixture of 2 parts by weight of ethylene glycol and 1 part by weight of water, dripped onto a TEM grid and dried.

[0034] The terms “spherical” or “ball-shaped” mean that the ratio of the particle length to the particle width is of 3:1 to 1:1, preferably 2:1 to 1:1, more preferably 1.5:1 to 1:1.

[0035] The proportion of secondary particles (hard, irreversibly agglomerated primary particles), using the process according to the present invention, is ≦20 wt. %, preferably ≦5 wt. %, preferably 2 wt. %, with respect to the total amount of precipitated zinc oxide. To determine this proportion, a 10 wt. % strength ZnO dispersion can be prepared in the same way as in working Example 6, stored for 5 days at room temperature and then filtered through a 0.2 μm cellulose membrane filter. The filter residues can be dried and weighed. The proportion of agglomerates can then be obtained by dividing the amount of solid determined in that way by the amount of ZnO used for the determination.

[0036] Mixtures of methanol with organic solvents and/or water can be used as solvents in the process according to the present invention. Methanol/water mixtures are preferably used. The use of methanol with less than 1 wt. %, preferably less than 0.5 wt. % of water is preferred. The use of methanol-free solvent systems does not lead to the formation of zinc oxide particles within the scope of the present invention.

[0037] The zinc acetate solution of the zinc salts corresponding to formula (I)

[0038] used in the process according to the present invention can be obtained by simple dissolution of the commercially available zinc salt, in a methanolic solvent as described above. For economic reasons, coarsely divided zinc oxide can also be initially introduced into a methanolic solvent and can be converted into a zinc salt solution by simply adding the corresponding acid and optionally water.

[0039] The residue R in formula (I) represents preferably H or an aliphatic or cycloaliphatic residue, more preferably H, CH₃, C₂H₅ or C₃H₇.

[0040] Most preferably zinc acetate optionally in form of its dihydrate can be used as zinc salt in the process of the present invention.

[0041] These solutions have a zinc ion concentration of preferably 1 to 2 mol per kg of solution.

[0042] The hydroxide solutions A2) or B2) have a concentration of hydroxide ions of preferably 3 to 6 mol per kg of solution.

[0043] Basically, a methanolic or aqueous sodium hydroxide solution, which may optionally contain dissolved potassium hydroxide, can also be used instead of a methanolic potassium hydroxide solution. The use of KOH-containing precipitation media (KOH and NaOHIKOH-containing solutions), however is advantageous as compared with pure NaOH-containing precipitation media and is therefore normally preferably used.

[0044] For the precipitation reaction, the zinc salt solution A1) or B1) is kept at a constant temperature of 30 to 65° C., preferably 50 to 65° C., more preferably 55 to 58° C.

[0045] The hydroxide solutions A2) or B2) have a temperature of 10 to 65° C., preferably 15 to 30° C., more preferably 18 to 25° C.

[0046] Preferably the molar ratio of base (OH) to Zn is 1.7-1.8, more preferably 1.72-1.78. Basically, ratios of less than 1.7 or more than 1.8 are also possible. Even when the ratio is <1.7, particles with the initially described sizes, shapes and properties can be obtained, but the amount of Zn which is not converted to ZnO is too large (poor space-time yield) so the process becomes unnecessarily costly and this embodiment is not preferred. When the ratio is >1.8, preferably >1.9, precipitation can no longer be controlled in such a way that particles with the initially described shapes and properties are obtained. Therefore a ratio >1.8 may not be preferred.

[0047] Basically, to control particle growth and particle morphology (the shape and surface structure), substances can also added before, during or after precipitation. These may be alkoxysilanes such as tetraethyl orthosilicate, zwitterionic compounds such as “betaine” (carboxytrimethylammonium) or 6-aminohexanoic acid or surface-active substances from colloid chemistry which are known to a person skilled in the art. Anionic, cationic or non-ionic surfactants, emulsifiers and/or stabilizers with carboxylate, sulfonate, ammonium or polyether groups may be preferred.

[0048] The process according to the present invention is normally performed at atmospheric pressure (1013 mbar), but may also be performed at higher or lower pressures. The process temperatures should then be adjusted accordingly so that they are not above the boiling point of the lowest-boiling component in the process. For the upper limit of 65° C. for the process temperature (corresponding to the boiling point of methanol at that pressure), this means that if the pressure is >1013 mbar the upper limit for the process temperature, corresponding to the boiling point of methanol, may be raised to the boiling point of methanol at the pressure then prevailing, or in the event of a lower pressure, has to be lowered accordingly. An upper limit of >65° C. for the temperature during the process is therefore possible.

[0049] For blending purposes, all stirring and mixing techniques included in the prior art can be used in the batch process, the use of radial and horizontal mixers such as cross-arm paddle mixers and MIG stirrers with aligned blades in combination with flow spoilers being preferred.

[0050] The energy input for the stirrers is generally 0.1 to 3 Watts per liter, preferably 0.2 to 0.8 Watts per liter.

[0051] Solution A2) is preferably added to A1) within less than 6 minutes, preferably within less than 4 minutes, more preferably within less than 3 minutes.

[0052] The temperature during the precipitation process is preferably, ≦65° C., more preferably, ≦60° C., most preferably 55 to 60° C.

[0053] The subsequent maturation process can be performed by continuing to stir the precipitation solution at preferably 50 to 65° C., more preferably 55 to 60° C. for preferably 20 to 40 min, more preferably 30 to 35 min.

[0054] The lower limit of 40° C. cited as the lower limit for the maturation temperature in all the process variants is the temperature above which maturation proceeds satisfactorily and above which therefore the maturation process is also normally performed. However, this does not exclude the use of maturation temperatures of ≦40° C.

[0055] Subsequent cooling down is achieved by the use of external cooling with a suitable medium such as cold water or brine, wherein the final temperature is preferably 10 to 25° C., more preferably 15 to 20° C., which means that further growth of the particles to give primary particles >50 nm, the formation of particles with e.g. rod-shaped morphology and/or agglomeration to give secondary particles, is prevented. The cooling process for the entire precipitation solution preferably takes less than 60 min.

[0056] In the continuous process, the two solutions B1) and B2) can be continuously brought together in a mixing unit of the static mixer type, or a T- or Y-junction piece, optionally with a downstream mixing section, and blended. The mixing units and the rates of flow of the reactant solutions or the precipitation solution are designed in such a way that homogeneous blending is achieved within preferably less than 6, preferably less than 4, more preferably less than 3 seconds. The time refers to a volume increment which is formed at time t =0 during the combination of a) and b) having a uniform density after preferably t <6, more preferably <4, most preferably <3 seconds.

[0057] The amount delivered of solutions B1) and B2) is adjusted in the process according to the present invention in such a way that, with the given starting temperatures of B 1) and B2), the temperature of the precipitation solution is preferably, ≦65° C., more preferably, ≦60° C. and more preferably 55 to 60° C. and the criterion for mixing time given above is complied with.

[0058] Subsequent particle maturation can be performed by passing the precipitation solution through a residence time section which is kept at a constant temperature of 50 to 65° C., preferably 55 to 60° C., wherein the latter is designed in such a way that the residence time for a volume increment of precipitation solution is preferably 20 to 40 min, more preferably 30 to 35 min. Subsequent cooling down to preferably 10 to 25° C., more preferably 15 to 20° C. can be performed by e.g. collecting the precipitation solution emerging from the residence time section in a tank with adequate jacket cooling or via heat exchangers from the prior art which are known per se to a person skilled in the art. Alternatively, a residence time section is not used, wherein the entire precipitation solution being produced after the blending procedure is collected in a stirred tank cooled to ≦20° C., preferably, ≦10° C. (effective internal temperature) and, after a desired time, this is heated with stirring to preferably 50 to 65° C., more preferably 55 to 60° C., without the further addition of any precipitation solution, and is matured for preferably 20 to 40 min, more preferably 30 to 35 min (semi-continuous process).

[0059] Also possible is precipitation in a stirred-tank under the conditions given for the batch process, wherein, however, the precipitate being formed is continuously withdrawn and either collected in a second tank at a temperature of ≦20° C., preferably ≦10° C. (effective internal temperature) and then matured (see semi-continuous process) or is continuously matured in a suitably designed stirred tank cascade.

[0060] Subsequent cooling down is achieved by external cooling with a suitable medium such as cold water or brine, wherein the final temperature is preferably 15 to 25° C., more preferably 15 to 20° C. The cooling process for the entire precipitation solution preferably takes less than 60 min.

[0061] According to the present invention, dissolved substances can be removed from the zinc oxide precipitates prepared and matured in accordance with the present invention and the precipitates are concentrated down by sedimentation in a centrifuge or under the effects of gravity or by cross-flow membrane filtration (nanofiltration or ultrafiltration using ceramic membranes having an average pore diameter of 5 nm build in multi-channel elements).

[0062] From the zinc oxides or their precipitates prepared by the process according to the present invention can be prepared dispersions in a variety of organic or aqueous solvents or mixtures, optionally with the aid of mechanical or chemical dispersers such as ionic or non-ionic surfactants, and/or surface-modifying compounds such as alkanoic acids and alkanoates with preferably 3 to 25 carbon atoms such as e.g. oleic acid, amines, aminoalcohols, alkoxysilanes or the products of hydrolysis of one or more alkoxysilanes by aqueous acids.

[0063] The dispersions mentioned above are prepared by stirring the methanolic zinc oxide precipitates obtainable in accordance with the present invention into organic and/or aqueous solvents or mixtures of these, optionally with the aid of surface-modifying substances.

[0064] According to the present invention, the methanol present in the dispersions can be removed by distillation in order to improve the dispersion status of the particles.

[0065] According to the present invention, water, monoalcohols, diols, aminoalcohols, alkanes, ethers, esters and also mixtures thereof can be used as solvents.

[0066] The use of halogenoalkanes and halogenoalkane/alcohol mixtures, such as dichloromethane/methanol and chloroform/methanol mixtures, are also preferred.

[0067] The use of alkanoic acids and alkanoates with preferably 3 to 25 carbon atoms, such as e.g. oleic acid, amines, preferably alkyl-, dialkyl- or trialkylamines, as stabilizers enables the zinc oxide particles prepared in accordance with the present invention also to be provided as a stable finely divided dispersion in non-polar solvents such as oils, alkanes and/or aromatic compounds.

[0068] Preferably, 10 wt. % of water, with respect to the ZnO present (dry weight), is added, with stirring, to a zinc oxide precipitate prepared in accordance with the invention and then a mixture of a monoalcohol, preferably n-butanol, and at most 5 wt. % of triethanolamine is added and after stirring for 30 minutes the methanol present is removed by distillation.

[0069] The solids concentration of the zinc oxide precipitates used to formulate dispersions is typically 5 to 80 wt. %, preferably 15 to 40 wt. %. The conductivity of the methanolic phase in these precipitates is less than 200 mS/cm, preferably less than 15 mS/cm, more preferably 0.005 to 5 mS/cm.

[0070] To improve the degree of dispersion of the particles, mechanical homogenization processes from the prior art can be used, with instruments such as high-speed stirrers (e.g. IKA-Ultra-Turrax® T25 basic, IKA-Werke GmbH & Co KG, D-79219 Staufen), ultrasonic dispersers (e.g. UP200S, UP400S, Dr. Hielscher GmbH, D-14513 Berlin) and/or jet dispersers (Chem. Ing. Tech. (69), 6/97, p. 793-798; EP-A 0 766 7997) being used.

[0071] The present invention also provides zinc oxides obtainable by the process according to the present invention and also dispersions prepared from same.

[0072] The use of these dispersions of primary particulate redispersed zinc oxides includes the preparation of molded items and/or coatings, for example those with UV-absorbing and/or a biocidal effect. Coatings are understood to be either polymeric systems for the coating of or adhesion of materials such as metals, plastics or glass or else cremes, salves, gels or similar solid or free-flowing formulations for use in the cosmetic or pharmaceutical area. Zinc oxides according to the present invention can also be used in plastics, rubbers, sealant compositions or adhesive compositions as fillers and/or additives with e.g. an acid-binding or catalytic effect.

[0073] The use of zinc oxides according to the present invention as vulcanization coactivators in rubbers and/or latex molded items is preferred.

[0074] The redispersible nanoparticulate zinc oxides according to the present invention can, as mentioned, be used as vulcanization coactivators during the preparation of latices based on natural and synthetic rubbers of all kinds.

[0075] Suitable rubbers which can be used to prepare latices include, apart from the wide variety of different natural latex formulations, synthetic rubbers such as natural latex and synthetic polyisoprenes, acrylonitrile/butadiene copolymers optionally containing carboxylated and/or self cross-linking groups, styrene/butadiene copolymers optionally containing carboxylated and/or self cross-linking groups, acrylonitrile/butadiene/styrene copolymers optionally containing carboxylated and/or self cross-linking groups, and optionally carboxylated chlorobutadiene latices. However, natural latex, carboxylated acrylonitrile/butadiene copolymers and chlorobutadiene latices as well as carboxylated chlorobutadiene latices are preferred.

[0076] During the vulcanization of different rubber latices, zinc oxide dispersions according to the invention are used in amounts of 2.0 to 0.01, preferably 0.5 to 0.05 parts by weight, with respect to 100 parts by weight of a latex mixture (dry wt./dry wt.) during vulcanization.

[0077] The zinc oxide dispersions used have a ZnO content of typically 5 to 40 wt. %, preferably 15 to 25 wt. %, wherein any aqueous medium, preferably mixtures of ethylene glycol/water, triethanolamine/water or ethylene glycol/water/triethanolamine, are suitable as the dispersion medium. The ratio by weight of ethylene glycol to water is preferably 5:1 to 1:1, more preferably 2.5:1 to 1.5 1. The ratio by weight of triethanolamine amine to water is preferably 1:5 to 1:1, more preferably 1:2.5 to 1:1.5. The ratio by weight of ethylene glycol to water to triethanolamine is preferably 10:5:5 to 10:5 0.1, more preferably 10:5:2 to 10:5:0.5.

EXAMPLES

[0078] The concentration of zinc oxide was determined in a similar way to that described in DE-A 199 07 704, by UV spectroscopic absorption measurements or, after dissolving the zinc oxide with glacial acetic acid or ammonia, by a volumetric titration with EDTA, using indicator buffer tablets.

Example 1 Preparation of Nano-ZnO from Zinc Oxide

[0079] 240.35 g of zinc oxide (tech. grade 99.8 wt. %) were initially introduced into 1320 g of methanol (tech. grade 99.9 wt. %) and heated to a steady 50° C. The solid was dissolved by adding 355.74 g of glacial acetic acid (tech. grade 99.9 wt. %) and 51.15 g of fully deionized water and the mixture was then heated to a steady 55° C. In order to remove any undissolved ZnO, a total of 34.36 g of KOH (tech. grade 90.22 wt. %) were added in 3 portions. Stirring was continued for 40 minutes and then a solution of 290.00 g of KOH (tech. grade 90.22 wt. %) in 660.00 g of methanol were added over the course of 2 minutes. The reaction temperature was 60° C. during the entire precipitation process. After a maturation time of 35 min, the reaction mixture was cooled to room temperature within 10 minutes by means of external cooling with ice.

Example 2 Preparation of Nano-ZnO from Zinc Acetate Dihydrate

[0080] 19800 g of methanol were initially introduced into a tank, under an atmosphere of nitrogen, and 9626 g of KOH were added carefully in portions with external water cooling. After stirring for a further 60 min, the dissolution process was terminated. In a second tank, 19338 g of zinc acetate dihydrate in 39600 g of methanol were initially introduced, at room temperature and under inert conditions. Then, under intensive stirring (MIG stirrer, 0.5 W/I), the mixture was heated to 50° C. and stirred for 30 min. After 60 min, the internal temperature had risen to 55° C. and 3130 g of the KOH/MeOH solution already made up was added to the vigorously stirred zinc acetate solution. The mixture than became largely clear. After a total of 30 min after adding the KOH, the actual precipitation process was initiated by transferring the major amount of the methanolic KOH solution over the course of 5 min. The temperature then rose to 58° C. After completion of the transfer process, the mixture was heated to 60° C. and the temperature was held for 35° C. Then the mixture was cooled to 20° C. by means of external water-cooling.

Example 3 Purification and Concentration of a ZnO Precipitation Prepared According to Example 1

[0081] The particles were compacted by sedimentation of the ZnO particles in the precipitate prepared according to example 1 for a period of 12 hours under the effects of gravity, then the clear methanolic supernatant liquid was removed from above via a lance using an attached pump, 550 g of fresh methanol were added with stirring and then the particles were allowed to settle out for a further 12 hours. The procedure was repeated a further 4 times, until the conductivity of the methanol removed from above was 1.9 mS/cm. The compacted zinc oxide precipitate had a ZnO content of 37.0 wt. %.

Example 4 Preparation of a 6-Aminohexanoic Acid-Stabilized Hydrosol

[0082] 54.1 g (ZnO content: 37.0 wt. %, corresponding to 20 g of ZnO) of the washed precipitate from example 3 were dispersed with a solution of 1 g of 6-aminohexanoic acid in 200 g of water. Then the total amount of dispersion was concentrated down to 200 g using a rotary evaporator at 45° C. bath temperature and 100 mbar pressure, wherein a translucent long-term stable dispersion of primary particulate redispersed particles was obtained.

Example 5 Preparation of a Sol in Triethanolamine/Water

[0083] 54.1 g (ZnO content: 37.0 wt. %, corresponding to 20 g of ZnO) of the washed precipitate from example 3 were dispersed in 180 g of a mixture of triethanolamine and distilled water. Then the total amount of dispersion was concentrated down to 200 g using a rotary evaporator at 45° C. bath temperature and 100 mbar pressure, wherein a translucent long-term stable dispersion of primary particulate redispersed particles was obtained.

Example 6 Preparation of a Sol in Ethylene Glycol/Water/Triethanolamine

[0084] 62.5 g (ZnO content: 37.0 wt. %, corresponding to 20 g of ZnO) of a ZnO precipitate according to example 3 were dispersed in 180 g of a mixture of ethylene glycol/water/triethanolamine (ratio 10:5:1). Then the total amount of dispersion was concentrated down to 200 g using a rotary evaporator at 45° C. bath temperature and 100 mbar pressure, wherein a translucent long-term stable dispersion of primary particulate redispersed particles was obtained.

Example 7 Preparation of a Sol in Ethylene 2Glycol/Water

[0085] 62.5 g (ZnO content: 32.0 wt. %, corresponding to 20 g of ZnO) of a washed precipitate according to example 3 were dispersed in 180 g of a 2:1 mixture of ethylene glycol/water. Then the total amount of dispersion was concentrated down to 200 g using a rotary evaporator at 45° C. bath temperature and 100 mbar pressure, wherein a translucent long-term stable dispersion of primary particulate redispersed particles was obtained.

Example 8 Preparation of an Organosol in CH₂Cl₂

[0086] A ZnO precipitate prepared according to example 1 was allowed to settle out for 4 hours. The supernatant liquid (2043 g, conductivity 24.3 mS) was removed from above and the residue was stirred for 30 min with 600 g of methanol. The dispersion of primary particulate redispersible particles was then centrifuged for 30 min at 5500 rpm in a laboratory centrifuge (Haraeus Variofuge RF, rotor radius 20.4 cm). The transparent supernatant liquid (837 g, conductivity 15.7 mS) was decanted off. The solid residue (263.1 g) was redispersed with 263.1 g of dichloro-methane. The dispersion of primary particulate redispersed ZnO particles made up had a weight of 508.3 g. After settling out for 72 h, this mixture was centrifuged for 30 min at 5500 rpm and pressure filtered through a 1 Am filter. A translucent, long-term stable dispersion of primary particulate redispersed particles was produced.

Example 9 Preparation of an Organosol in Butanol/triethanolamine

[0087] 28.4 g of a 4 wt. % strength solution of triethanolamine in n-butanol were added, with stirring, to 71.6 g of a precipitate prepared according to example 1 and washed according to example 3 (34.8 wt. % ZnO, conductivity of the liquid phase 3 mS/cm). To improve the degree of dispersion of the primary particles, the dispersion obtained was homogenized by a single treatment with a nozzle jet disperser at 400 bar. A translucent, long-term stable dispersion of primary particulate redispersed particles was produced.

Example 10 Redispersible Zinc Oxides in Latex Molded Parts

[0088] Use of the dispersion obtained from example 6 to produce latex molded parts.

[0089] 167 g of a natural latex of the-HA type (according to the ISO 2004 specification) was mixed, at room temperature with stirring, with 5.0 parts by weight of a 10 wt. % strength aqueous potassium hydroxide solution and with 0.70 parts by weight of a 20 wt. % strength potassium laurate solution as stabilizer. Then 20.6 parts by weight of a vulcanization paste, consisting of 1.5 parts by wt. of colloidal sulfur, 0.6 parts by wt. of zinc dithiocarbamate (ZDBC), 1.5 parts by wt. of zinc mercaptobenzothiazole (ZMBT), 1.5 parts by wt. of an anti-ageing agent based on phenol and 15.5 parts by wt. of a 5 wt. % strength solution of a disperser consisting of an Na salt of a condensation product of naphthalinesulfonic acid and formaldehyde, were added. In addition, 0.25 parts by wt. of a commercially available colophonium wax emulsion (Michemlube® 124, (Michelman, Inc., 9080 Shell Road, Cincinnati, Ohio 45236-1299 USA)) were added.

[0090] The solids content of this latex compound was 56 wt. %.

[0091] The data relating to parts by wt. (parts by weight) refer to 100 parts by weight of dry rubber substance, which corresponds to 167 parts by weight of wet natural latex.

[0092] The vulcanization activator ZnO was then added in two different ways:

[0093] a) the corresponding amount of zinc oxide was added directly following the previous steps or

[0094] b) this was only added 2 hours after the previous steps, with stirring.

[0095] Amount of Zinc Oxide Added:

[0096] Trial series 10-A with 0.05 parts by wt. of zinc oxide, prepared as in example 6 (according to the invention)

[0097] Trial series 10-B with 2.0 parts by wt. of zinc oxide, white sealer (surface area 10 m²/g; manufactured by Grillo Zinkoxid GmbH, Germany; powdered form) (comparison)

[0098] Trial series 10-C with 1.0 parts by wt. of active zinc oxide (surface area at least 45 m²/g; manufactured by Bayer AG, Germany; powdered form) (comparison)

[0099] Following this, maturation was performed for 48 hours in each case with constant stirring at 50 rpm and a temperature of 40° C. This matured compound was then filtered through a 100 μm filter in order to remove any coagulated material. After allowing the filtered latex to stand for 10 minutes, dip molding was performed to prepare specimen molded parts. Here, lightly sand-blasted glass plates (dimensions 100×180×4 mm) were dipped into a 15 wt. % strength calcium nitrate solution, as coagulant solution, for 10 seconds and dried. A film deposition of about 0.30 mm was achieved in this way. The films prepared in this way were then dried at 80° C. in hot air for a period of 30 min and then vulcanized at 120° C. for 15 min.

[0100] After a conditioning phase of 24 hours in a normal atmosphere, the non-aged specimen molded parts obtained were subjected to materials testing, wherein the modulus (M300: modulus at 300% extension), the strength (F-max) and the extension at break (Break ext.: extension up to the break point) were determined (Zwick test equipment measured in accordance with DIN 53 504, specimen molded part S2, and ISO 37). TABLE 1 Strength values without thermal ageing M300 F-max Break ext. Example [MPa] [MPa] [%] 10-A a) 1.7 21.9 774 10-A b) 1.5 24.9 832 10-B a) 2.0 18.3 619 10-B b) 1.5 21.6 774 10-C a) 2.3 15.7 569 10-C b) 2.4 14.1 552

[0101] The results show that the zinc oxide prepared according to the present invention provides comparable strength values, despite a lower amount being used, to those obtained with the use of 2.0 parts by wt. of white sealer zinc oxide or 1.0 parts by wt. of a zinc oxide with a high surface area. The modulus at 300% extension is much lower when using the zinc oxide prepared according to the present invention than when using comparison samples with zinc oxides which are not according to the present invention. This effect leads to greater wearer comfort which is of importance, for example, when producing latex gloves. The extension up to the break point in the case of trial series 10-A according to the invention also gives higher values than the comparison tests 10-B and 10-C.

[0102] To assess the resistance to ageing, the specimen molded parts were then stored for 8 and 16 h in hot air at 100° C. and the strength values mentioned above were determined again. TABLE 2 Strength values after thermal ageing, 8 h at 100° C. M300 F-max Break ext. Example [MPa] [MPa] [%] 10-A a) 1.7 18.6 745 10-A b) 1.5 23.2 816 10-B a) 2.5 15.4 566 10-B b) 1.4 20.5 798 10-C a) 2.8 11.8 510 10-C b) 2.8 12.1 495

[0103] TABLE 3 Strength values after thermal ageing, 16 h at 100° C. M300 F-max Break ext. Example [MPa] [MPa] [%] 10-A a) 1.7 17.3 728 10-A b) 1.6 21.4 789 10-B a) 2.6 13.2 526 10-B b) 1.4 18.7 785 10-C a) 2.9 10.1 478 10-C b) 2.8 6.2 453

[0104] Assessment after ageing showed clear improvements in stability after 8 and 16 hours storage in hot air at 100° C. in the case of trial series 10-A a) and b) (better resistance to ageing).

Example 11

[0105]167 g of a natural latex of the HA type (according to the ISO 2004 specification) were mixed with 5.0 parts by wt. of a 10 wt. % strength aqueous potassium hydroxide solution and with 1.25 parts by weight of a 20 wt. % strength potassium laurate solution as stabilizer, at room temperature and with stirring. Then 7.8 parts by weight of a vulcanization paste consisting of 1.0 parts by wt. of colloidal sulfur, 0.6 parts by wt. of zinc dithiocarbamate (ZDBC), 0.3 parts by weight of zinc mercaptobenzothiazole (ZMBT), 1.0 parts by wt. of an anti-ageing agent based on phenol were added and 4.9 parts by wt. of a 5 wt. % strength aqueous solution of a disperser consisting of the Na salt of a condensation product of naphthalinesulfonic acid and formaldehyde were also added.

[0106] Then the stated amounts of zinc oxide were added, with stirring.

[0107] 11-A: 0.05 parts by wt. of a zinc oxide prepared according to example 6 and

[0108] 11-B: 0.05 parts by wt. of a zinc oxide prepared according to DE-A 199 07 704

[0109] 11-C: 1.0 parts by wt. of zinc oxide WS (surface area 10 m²/g, manufactured by Grillo Zinkoxid GmbH, Germany; powdered form), used as a 50 wt. % strength aqueous paste

[0110] 11-D:0.5 parts by wt. of active zinc oxide (surface area at least 45 m²/g;

[0111] Bayer AG, Germany; powdered form) used as a 5 wt. % strength aqueous paste.

[0112] These mixtures were then adjusted to a solids content of 45 wt. % by adding water. Following this, the maturation process was performed in each case for 48 hours with constant stirring at 50 rpm and at a temperature of 40° C. This matured compound was then filtered through a 100 μm filter in order to remove any coagulated material. After allowing the filtered latex to stand for 10 minutes, dip molding was performed to prepare specimen molded parts. Here, glass plates (as described in example 10) were dipped into a 15 wt. % strength calcium nitrate solution, as coagulant solution, for 10 seconds and dried. A film deposition of about 0.30 mm was achieved in this way. The films prepared in this way were then dried at 80° C. in hot air for a period of 30 min and then vulcanized at 120° C. for 15 min.

[0113] After a conditioning phase of 24 hours in a normal atmosphere, the non-aged specimen molded parts obtained were subjected to materials testing, wherein the moduli (M300 and M700: modulus at 300 and 700% extension respectively), the strength (F-max) and the extension at break (Break ext.: extension in % up to the break point) were determined (Zwick test equipment measured in accordance with DIN 53 504, specimen molded part S2, and ISO 37). TABLE 4 Strength values without thermal ageing M300 M700 F-max Break ext. Example [MPa] [MPa] [MPa] [%] 11A 1.1 7.1 23.6 934 11B 1.4 8.6 22.1 857 11C 1.5 13.4 26.2 829 11D 1.6 19.1 21.7 725

[0114] The nanoparticulate zinc oxide according to example 6, according to the invention, exhibits a higher strength, a lower modulus and a higher extension at break in comparison to the nanoparticulate zinc oxide already described but also as compared with the types of zinc oxide used in practice.

[0115] To assess the resistance to ageing, the specimen molded parts were then stored in hot air at 100° C. for 8 h and 16 h and the strength values mentioned above were determined again: TABLE 5 Strength values after thermal ageing, 8 h at 100° C. M300 M700 F-max Break ext. Example [MPa] [MPa] [MPa] [%] 11A 1.2 6.7 23/6 923 11B 1.5 12.1 18.7 764 11C 1.5 12.1 22.7 797 11D 1.7 17.2 21.2 735

[0116] TABLE 6 Strength values after thermal ageing, 16 h at 100° C. M300 M700 F-max Break ext. Example [MPa] [MPa] [MPa] [%] 11A 1.1 5.9 21.4 921 11B 1.5 10.7 17.3 770 11C 1.6 17.2 21.8 734 11D 1.8 18.8 669

[0117] The improved resistance to ageing was also exhibited after the ageing process. This was expressed in particular by the virtual absence of losses in strength as compared with all the other types of zinc oxide and especially in that no increases in the modulus values were noted. The extension at break determined was virtually unaltered, whereas the extension at break decreased clearly, by up to 100%, with the types of zinc oxide used for comparison purposes.

[0118] Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims. 

What is claimed is:
 1. A process for the batchwise preparation of zinc oxide particles comprising A1) treating a methanolic solution of zinc salts corresponding to formula (I)

 with a concentration of zinc ions (Zn) of 0.01 to 5 mol per kg of solution with A2) a methanolic potassium hydroxide solution with a concentration of hydroxide ions (OH) of 1 to 10 mol per kg of solution in a molar ratio of OH to Zn of 1.5 to 1.8 with stirring, wherein the precipitation solution is matured at a temperature of 40 to 65° C. for a period of 5 to 50 min and is then cooled down to a temperature of ≦25° C.
 2. A process for the continuous preparation of zinc oxide particles comprising B1) mixing a methanolic solution of zinc salts corresponding to formula (I)

 with a concentration of zinc ions (Zn) of 0.01 to 5 mol per kg of solution and B2) a methanolic potassium hydroxide solution with a concentration of hydroxide ions (OH) of 1 to 10 mol per kg of solution in a mixing unit wherein b1) the molar ratio of OH to Zn is 1.5 to 1.8, b2) mixing is homogeneous, and the precipitation solution formed is matured at a temperature of 40 to 65° C. for a period of 5 to 50 min and is then cooled down to a temperature of ≦25° C.
 3. A process for the batchwise preparation of zinc oxide particles comprising A1) treating a methanolic of zinc salts corresponding to formula (I)

 solution with a concentration of zinc ions (Zn) of 0.01 to 5 mol per kg of solution with A2) a sodium hydroxide solution, which may optionally contain dissolved KOH, with a total concentration of hydroxide ions (OH) of 1 to 10 mol per kg of solution in a molar ratio of OH to Zn of 1.5 to 1.8 with stirring, wherein, the precipitation solution obtained is matured at a temperature of 40 to 65° C. for a period of 5 to 50 min and is then cooled down to a temperature of ≦25° C.
 4. A process for the continuous preparation of zinc oxide particles comprising B 1) mixing a methanolic solution of zinc salts corresponding to formula (I) with a concentration of zinc ions (Zn) of 0.01 to 5 mol per kg of solution and B2) a sodium hydroxide solution, which may optionally contain dissolved KOH, with a total concentration of hydroxide ions (OH) of 1 to 10 mol per kg of solution in a mixing unit wherein b1) the molar ratio of OH to Zn is 1.5 to 1.8, b2) the mixture being formed in the mixing unit is blended homogeneously, and, the precipitation solution formed is matured by keeping at a temperature of 40 to 65° C. for a period of 5 to 50 min and is then cooled down to a temperature of ≦25° C.
 5. A process according to claim 1, wherein the molar ratio of OH:Zn is 1.7-1.8 and the methanolic zinc salt solution is a methanolic solution of zinc acetate.
 6. Zinc oxide particles prepared by a process according to any of claims 1 to
 5. 7. Dispersions comprising zinc oxide particles according to claim
 6. 8. Dispersions according to claim 7, having azinc oxide content of 5 to 40 wt. %.
 9. Dispersions according to claim 7, wherein the dispersion comprises water mixed with ethylene glycol and/or triethanolamine.
 10. A filler and/or additive comprising zinc oxide particles according to claim
 6. 11. Vulcanization coactivator comprising zinc oxide particles according to claim
 6. 12. UV absorbers comprising zinc oxide particles according to claim
 6. 13. Fungicides and/or biocides comprising zinc oxide particles according to claim
 6. 14. Coatings and molded parts comprising zinc oxide particles according to claim
 6. 